In the processing of materials containing metal values, two of the main extractive methods to be considered are pyrometallurgy and hydrometallurgy. In the former, metal-containing material such as ore, slag, scrap, etc. is heated with appropriate agents such as reducing agents, fluxing agents, sulfidizing agents, chloridizing agents and/or oxidizing agents, etc., usually to the melting or fusion point of the mixture. At this temperature there is generally a separation of metallic values from gangue or waste materials. The procedure then calls for separating the metallic values from slag or waste material at a temperature at which both are molten. The phase containing the metal value is then cast to some convenient shape for use or for further refining, whichever is appropriate for the particular system involved. The very high temperatures involved in this technique are achieved via electric furnaces, blast furnaces, reverberatory furnaces, etc. Temperatures required for metals such as copper, nickel, iron would generally range from 2000.degree. to 3000.degree. F. An advantage in this method is that recoveries of the metal values are typically quite high.
The hydrometallurgy approach differs substantially from pyrometallurgy in that, although the metal bearing material such as ore, slag, scrap, etc. may be heated with agents such as reducing agents, oxidizing agents, sulfidizing and chloridizing agents as part of the procedure, the temperatures involved are generally much lower than with the usual pyrometallurgical method. These temperatures typically may be 500.degree. to 1900.degree. F., temperatures generally well below the fusion point of the metal-containing material.
Following this step, the treated metal-containing material then is contacted with an appropriate aqueous solution for extracting metal values by dissolution. The metal is then removed from the solution by precipitation, solvent extraction, evaporation of solvent, etc. The metal-containing residue obtained is then handled appropriately to further refine the metal. Although conditions of temperature are generally much lower than in pyrometallurgy, it is frequently found that recovery of the metal values is also lower than in the pyrometallurgical method.
A particular case where this is true concerns the extraction of nickel from lateritic nickel ores. The pyrometallurical processes range from the use of an electric furnace for the direct smelting of ore to produce ferronickel through similar techniques involving the blast furnace in which an iron-nickel-sulfide matte is obtained. The extraction of nickel from the ore using this method is greater than 90%.
Of the several hydrometallurgy approaches used commercially for treating this type of ore, the practice on a highly limonitic ore, such as at Nicaro, Cuba, involves roasting the ore in a multihearth furnace while a reducing gas such as producer gas passes countercurrent to the ore. Temperatures in this case range from about 900.degree. to about 1350.degree. F. Following the roasting step, the ore is cooled in the absence of air, mixed with an ammoniacal ammonium carbonate solution and vigorously agitated and aerated. This results in the dissolution of nickel and cobalt, separating them from the bulk of the ore. This solution then is treated with steam, driving off ammonia and precipitating nickel carbonate. This product then is treated further to obtain the appropriate form of nickel or used as such. In comparison to the pyrometallurgical process, however, extractions using this method have only been of the order of 70 to 80%.
Several other hydrometallurgy methods involve the use of procedures which include a roasting step with chlorides or sulfates, but in other than reducing atmospheres, and the roaster ore is leached with an appropriate solvent such as dilute sulfuric acid. Alternatively, in certain cases the ore can be leached directly, such as with sulfuric acid solution, but this is practical only when the magnesia content of the ore is low and when leaching conditions of high temperature, high pressure and high acid concentration are used. As will hereinafter be shown in greater detail, we have now discovered that an effective and economical process for recovering nickel using ambient leaching conditions and dilute acids may be effected by utilizing the process of the present invention.
In processes involving the reductive roast of metal-bearing sources, a major portion of the energy which is required in roasting metal sources such as ores is expended in removing water from the surface and from the lattices of the ore. Inasmuch as energy costs have increased at a tremendous rate during the past few years, thus substantially increasing the overall cost of recovering desired metal values from the metal-bearing source, it is necessary to discover new methods for the recovery of the metal values by utilizing processes which will greatly lessen the enegy cost of the process, and thus lower the overall amount of money which is expended to render the process economically attractive or viable to operate.
As will hereinafter be shown in greater detail, we have now discovered a method for the recovery of metal values from metal-bearing sources without the undesirable consumption of an excess amount of energy.